1. Field of the Invention
The present invention relates generally to the technical field of mobile communications, and specifically to mobile communications systems, communications apparatuses, and communications methods that use next-generation mobile communications techniques.
2. Description of the Related Art
In this type of technical field, mobile communications schemes to succeed so-called third generation mobile communications schemes are being studied by 3GPP, a standardization body for wideband code division multiple access (W-CDMA) schemes. More specifically, as the mobile communications schemes to succeed the W-CDMA, high-speed downlink packet access (HSDPA), high-speed uplink packet access (HSUPA) schemes, etc., not only Long term evolution (LTE) schemes, but also succeeding mobile communications schemes are being studied. Successors to an LTE system include, for example, IMT-Advanced, LTE-Advanced, and fourth-generation mobile communications systems, etc.
A downlink radio access scheme in the LTE system is orthogonal frequency division multiple access (OFDMA). For uplink, single-carrier frequency division multiple access (SC-FDMA) is used. However, a multi-carrier scheme may be used for uplink in a different system.
The OFDMA is a scheme which divides a frequency band into multiple narrow frequency bands (sub-carriers) and overlaying data onto the respective sub-carriers to transmit the overlaid data. Densely lining up the sub-carriers such that they are in an orthogonal relationship with each other on the frequency axis makes it possible to expect that high-speed transmission be achieved and frequency utilization efficiency be increased.
The SC-FDMA is a single-carrier transmission scheme which divides a frequency bandwidth for each terminal in the Fourier-transformed frequency domain and allows using mutually different frequency bands among multiple terminals. Such a scheme as described above, which makes it possible to easily and effectively reduce inter-terminal interference as well as to reduce fluctuations in transmission power, is preferable from points of view of reducing power consumption of terminals and widening the coverage, etc. The SC-FDMA scheme, which uses a DFT-spread OFDM scheme, for example, corresponds to mapping locations of a signal that are restricted to a series of contiguous frequency bands, or a signal mapped in the frequency domain to a comb-tooth shape in certain intervals. Using single-carrier FDMA in uplink is disclosed in Non-Patent document 1, for example.
In a system such as the LTE, for downlink or uplink, one or more resource blocks (RBs) or resource units (RUs) are allocated to a user apparatus to conduct communication. The resource blocks are shared by a large number of user apparatuses within a system. In the LTE, the base station apparatus determines which user apparatus of multiple user apparatuses a resource block is allocated to for each sub-frame, which is 1 ms. The sub-frame may be called a transmission time interval (TTI). Determining allocations of radio resources is called scheduling. In downlink, a base station apparatus transmits, to a user apparatus selected in the scheduling, a shared data channel in one or more resource blocks. The shared data channel is called a physical downlink shared channel (PDSCH). In uplink, the user apparatus selected in the scheduling transmits, to the base station apparatus, a shared channel in one or more resource blocks. The shared channel is called a physical uplink shared channel (PUSCH).
In a communications system using the shared channels as described above, it is necessary, in principle, to signal, for each sub-frame, which user apparatus the shared channel is allocated to. The control channel used in the above-mentioned signaling is called a physical downlink control channel (PDCCH) or a downlink (DL)-L1/L2 control channel (PDCCH). In addition to the PDCCH, a downlink control signal may include a physical control format indicator channel (PCFICH) and a physical hybrid ARQ indicator channel (PHICH).
The PDCCH may include the following information sets, for example: (See, Non-patent document 2, for example.)
a downlink scheduling grant;
an uplink scheduling grant;
an overload indicator; and
a transmission power control command bit.
Downlink scheduling information includes, for example, information on a downlink shared channel and more specifically includes information on allocating downlink resource blocks, information on identifying user apparatuses (UE-ID), number of streams, information on pre-coding vectors, data size, modulation schemes, information on HARQ (hybrid automatic repeat request), etc.
Moreover, the uplink scheduling grant includes, for example, information on an uplink shared channel and more specifically includes information on allocating uplink resources, information on identifying user apparatuses (UE-ID), data size, modulation schemes, information on uplink transmission power, information on demodulation reference signals in uplink MIMO, etc.
The PCFICH is information for reporting a PDCCH format. More specifically, the number of OFDM symbols mapped to the PDCCH is reported using the PCFICH. In the LTE, the number of OFDM symbols mapped to the PDCCH is 1, 2 or 3, the mapping being performed in order from a beginning OFDM symbol of a sub-frame.
The PHICH includes acknowledgement/non-acknowledgement information (ACK/NACK), which indicates whether retransmission is needed for the PUSCH transmitted in uplink. The PHICH, which indicates correct/not correct for each transmission unit such as a packet, may basically be represented in one bit. Therefore, it is not advantageous for radio transmission as it is. Therefore, PHICHs of a number of people are collected to construct information sets of multiple bits, the information undergoing multiplexing and spreading using code multiplexing and then being wirelessly transmitted.
For definition of terms, the PDCCH, PCFICH and PHICH may be defined as respectively independent channels in a downlink signal as described above, or the PDCCH may be defined to include the PCFICH and PHICH.
In uplink, user data (a normal data signal) and accompanying control information are transmitted using the PUSCH. Moreover, separately from the PUSCH, downlink quality information (CQI; channel quality indicator) and PDSCH acknowledgement/non-acknowledgement information (ACK/NACK), etc., are transmitted using the physical uplink control channel (PUCCH). The CQI is used for downlink shared physical channel scheduling process, adaptive modulation and coding scheme (AMCS), etc. In uplink, a random access channel (RACH) and a signal which indicates a request for allocating uplink and downlink radio resources, etc., is also transmitted as needed.
As shown in FIG. 1, the above-described acknowledgement information is transmitted a certain time period after receiving a data channel. In the illustrated example, the downlink data channel (DL data) is received at the user apparatus and is determined to be ACK or NACK, and the determined result is transmitted in uplink. It is desirable that acknowledgement information, which greatly affects throughput of data transmission, is transmitted accurately. From such a viewpoint, it is being proposed to repeatedly transmit acknowledgement information when communications conditions are not good.
FIG. 2 shows how a downlink data channel is received at a user apparatus and ACK or NACK is transmitted repeatedly (twice) in uplink. Such a repetition as described above is disclosed in Non-patent document 3.
Non-patent document 1:
3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006
Non-patent document 2:
3GP R1-070103, Downlink L1/L2 Control Signaling Channel Structure Coding”, Jan. 15-19, 2007
Non-patent document 3:
3GPP, R1-081950, “Necessity of ACK/NAK Repetition in PUCCH”, May 5-9, 2008.